Thermal transfer printing involves forming an image (in reverse) on a retransfer intermediate sheet using one or more thermally transferable dyes. The image is then thermally transferred to a surface of an article by bringing the image into contact with the article surface and applying heat and typically also pressure. Thermal transfer printing is particularly useful for printing onto articles that are not readily susceptible of being printed on directly, particularly three-dimensional (3D) articles. Thermal transfer printing by dye diffusion thermal transfer printing, using sublimation dyes, is disclosed, e.g., in WO 98/02315 and WO 02/096661. By using digital printing techniques to form the image on the retransfer intermediate sheet, high quality images, possibly of photographic quality, can be printed on 3D articles relatively conveniently and economically even in short runs. Indeed such articles can be personalised economically.
Using suitable retransfer intermediate sheets, it is possible to form good quality images on 3D articles, possibly having complex shapes including curved shapes (concave or convex) including compound curves. When printing onto 3D articles, the sheet is typically preheated, e.g. to a temperature in the range 80 to 170° C., prior to application to the article, to soften the sheet and render it deformable. The softened sheet is then in a condition in which it can be easily applied to and conform to the contours of an article. This is conveniently effected by establishing a pressure difference across the softened sheet to cause it to mould to the article. The pressure difference is typically 40 kPa, and is typically applied as a vacuum to that side of the sheet which is applied to the article. While the sheet is maintained in contact with the article, e.g. by maintenance of the pressure difference, the sheet, and also the article, are heated to a suitable temperature for dye transfer, typically a temperature in the range 140 to 200° C., for a suitable time, typically in the range 15 to 150 seconds. After dye transfer, the article is allowed or caused to cool before removal of the retransfer intermediate sheet. Suitable apparatus for performing the retransfer printing step is disclosed e.g. in WO 01/96123 and WO 2004/022354.
The pressure difference across the sheet causes a force to be exerted on the article, which, at the temperature to which the article is heated for dye transfer, can cause deformation of the article, especially if it is made of thin metal or plastics materials that soften on heating.
The article is therefore typically placed on a rigid support, referred to as a “nest”, that limits deformation of the article during thermal transfer printing.
The part of the support on which the article is placed typically forms a relatively loose fit with the article because of the need to accommodate manufacturing tolerances in the dimensions of the article and different rates of thermal expansion of the support and the article, such that there can be gaps between the support and the article.
Where there is a gap between the support and the article, deformation of the article can occur when the article is heated and a force is exerted on the article, despite the presence of the support. Moreover, where there is a gap between the support and an edge of the article, the softened sheet can be forced into the gap by the pressure difference across the sheet and can rupture, so that the pressure difference is lost. In that event the dye transfer from the sheet to the article is likely to be unsatisfactory.